etcd3gw: Discover API version automatically

etcd changed API version from v3beta to v3alpha, then v3, and because
of transition available api versions are different according to
the etcd server available.

This change implements the mechanism to detect the current api version
according to the etcd version obtained via the version API. So that
the url without api version works regardless of the etcd server used.

Closes-Bug: #2043810
Depends-on: https://review.opendev.org/c/openstack/etcd3gw/+/901244
Change-Id: Ib81390eb73feafe08d007db1ced4d0ceb3bde212

releasenotes/notes/etcd3gw-api-version-discovery-3e9943c73fae48d4.yaml

@@ -0,0 +1,11 @@
+---
+features:
+  - |
+    The ``etcd3gw`` driver now automatically discovers available etcd API
+    version and determines the api path. The driver detects the appropriate api
+    path before sending its first request, and the path is reused for its
+    subsequent requests. Services may need to be restarted after its backend
+    etcd server is upgraded, so that the new api path is detected.
+    The detection is skipped if the coordination backend url contains
+    the ``api_version`` query parameter or the ``ETCD3GW_API_PATH``
+    environment is set.

setup.cfg

@@ -53,7 +53,7 @@ etcd3 =
   grpcio>=1.18.0
   protobuf<4.0 # BSD License (3 clause)
 etcd3gw =
-  etcd3gw!=0.2.6,>=0.1.0 # Apache-2.0
+  etcd3gw!=0.2.6,>=2.3.0 # Apache-2.0
 zake =
   zake>=0.1.6 # Apache-2.0
 redis =

tooz/drivers/etcd3gw.py

@@ -181,7 +181,7 @@ class Etcd3Driver(coordination.CoordinationDriverCachedRunWatchers,
     ==================  =======
     Name                Default
     ==================  =======
-    api_version         v3
+    api_version         None
     ca_cert             None
     cert_key            None
     cert_cert           None
@@ -200,9 +200,6 @@ class Etcd3Driver(coordination.CoordinationDriverCachedRunWatchers,
     #: Default port used if none provided (4001 or 2379 are the common ones).
     DEFAULT_PORT = 2379
 
-    #: Default api version if none provided
-    DEFAULT_API_VERSION = "v3"
-
     GROUP_PREFIX = b"tooz/groups/"
 
     def __init__(self, member_id, parsed_url, options):
@@ -215,14 +212,18 @@ class Etcd3Driver(coordination.CoordinationDriverCachedRunWatchers,
         cert_key = options.get('cert_key')
         cert_cert = options.get('cert_cert')
         timeout = int(options.get('timeout', self.DEFAULT_TIMEOUT))
-        api_version = options.get("api_version", self.DEFAULT_API_VERSION)
+        api_version = options.get("api_version")
+        if api_version:
+            api_path = "/" + api_version + "/"
+        else:
+            api_path = None
         self.client = etcd3gw.client(host=host,
                                      port=port,
                                      protocol=protocol,
                                      ca_cert=ca_cert,
                                      cert_key=cert_key,
                                      cert_cert=cert_cert,
-                                     api_path="/" + api_version + "/",
+                                     api_path=api_path,
                                      timeout=timeout)
         self.lock_timeout = int(options.get('lock_timeout', timeout))
         self.membership_timeout = int(options.get(

tooz/tests/drivers/test_etcd3gw.py

@@ -34,8 +34,7 @@ class TestEtcd3Gw(testcase.TestCase):
                'ca_cert': None,
                'cert_key': None,
                'cert_cert': None,
-               'api_path': (
-                   "/" + etcd3gw_driver.Etcd3Driver.DEFAULT_API_VERSION + "/"),
+               'api_path': None,
                'timeout': etcd3gw_driver.Etcd3Driver.DEFAULT_TIMEOUT},
               {'coord_url': ('etcd3+https://my_host:666?ca_cert=/my/ca_cert&'
                              'cert_key=/my/cert_key&cert_cert=/my/cert_cert&'